The present invention relates to a display apparatus including an optical modulation element. More particularly, the invention relates to a display apparatus employing a luminance gradation modulation system.
In the recent years, there has been substantial progress in the reduction of thickness and weight of an image display apparatus. In place of a CRT, which has been a primary image display device, a flat panel display, such as a liquid crystal display, PDP (Plasma Display Panel) and ELD (Electroluminescent Display), has experienced rapid development.
On the other hand, concerning the performance of a display apparatus, in association with wide-spread use of the personal computer (PC), digital video disk (DVD) and digital television broadcasting, the provision of a display having high definition and high or multiple level gradation has become essential. Also, the demand for higher performance, particularly a higher level of definition in an image display apparatus, is expected to grow in the future.
The degradation of image quality when a wide dynamic image is displayed in a hold illumination type image display apparatus, such as a liquid crystal display device, has been reported in the Institute of Telecommunications Engineers Technical Report EID 96-4, pp. 19-26 (June, 1996). According to this report, due to the unmatching of a dynamic image in hold illumination and the radial motion of the human eye when following a dynamic image, bluing of the dynamic image can be caused, thereby to lower the image quality of the dynamic image display.
In the above-identified report, it has been indicated that a method of multiplying the frame frequency n times and other methods may eliminate or reduce any lowering of the image quality of the dynamic image display. In short, in order to attain a clear dynamic image in the hold illumination type display apparatus, such as a liquid crystal display device, the display frequency has to be made higher. However, as set forth above, in the current image display method or the driving system of an image display apparatus, the increasing of the display frequency is becoming close to the limit. Accordingly, for this fact, the foregoing method is difficult to realize in practice.
The conventional display method of displaying an image by rewriting at high speed corresponding to an increase of the display frequency has been disclosed in Japanese Patent Application Laid-open No. 11-75144 (1999), for example. In the disclosed display method, two memories and two kinds of means for a driving pixel according to the contents of the memories are provided per each pixel, including an optical modulation element. For all pixels forming a preliminarily displayed image, data is written in the first memory in each pixel. Subsequently, the contents of the first memories are transferred to the second memories all together simultaneously for effecting ON and OFF control of the light at each pixel according to data in the second memories at high speed for PWM (pulse width modulation) control for multiple level gradation image display.
The above-mentioned prior art encounters a problem in multiple-level gradation display performance since no consideration has been given to the necessity for provision of a high speed optical modulation element for each pixel. Namely, since the conventional display method obtains multiple level gradation display by PWM control, a high response speed is required for the optical modulation element used in each pixel.
In this regard, in the prior art, a ferroelectric liquid crystal or antiferroelectric liquid crystal, for example, is used for the optical modulation element. Such a liquid crystal device requires a difficult fabrication process, such as orientation control or gap adjustment. Also, since the electrostatic capacity thereof is relatively large, the drive control is difficult.
Furthermore, in PWM control, it is not possible to drive the display in a saturated luminance output (=all white display) condition over an entire period of one frame. Therefore, there is a limitation in the light using efficiency and illumination period efficiency, thereby making it difficult to attain a gradation display at a maximum value.